Conventional virgin aluminum production typically involves the reduction of alumina which has been dissolved in a cryolite-containing electrolyte. The reduction is carried out in a Hall-Heroult cell containing a carbon anode and a carbon cathode which also serves as a container for the electrolyte. When current is run through the electrolyte, liquid aluminum is deposited at the cathode while gaseous oxygen is produced at the anode. Because the heat flux of the cell must be strictly maintained, insulating materials, such as dense firebrick, are used under the carbon cathode container.
Although cryolite is quite useful in dissolving alumina, it nonetheless compromises the long term life of the cell. Upon electrolysis, the cryolite ionizes into highly reactive species, in particular, sodium and fluorine ions and aluminum radicals. Over time, these species pass through the carbon cathode containment and contact the firebrick. Upon contact, the species attack the firebrick and gradually degrade its physical integrity to the point where the firebrick must be replaced.
Previous attempts to solve this problem have met with mixed success. Vapor barriers such as steel sheets have been placed between the cathode and the firebrick. However, these sheets are eventually consumed by the cryolite and produce contamination. Glass sheets have been similarly placed in an attempt to produce a viscous liquid barrier between the cathode and the firebrick, with similar results. Other efforts have used castables and dry vibrated cements ("DVC") such as mullite as the protective barrier. Although these materials react with the cryolite to produce a relatively stable assemblage, the silica and alumina contained therein are highly susceptible to attack from the ionized cryolite species. One effort to improve upon high alumina castable barriers, U.S. Pat. No. 4,126,474, added barium to the castable. Although this effort improved cryolite resistance, barium is a heavy metal and so carries with it environmental concerns.
Accordingly, it is the object of the present invention to provide a dry vibrated cement ("DVC") or castable refractory which offers superior resistance to cryolite penetration without posing environmental concerns.